should you only download XCode 4.1 from the app store if you move to Lion?
i.e I am staying on Leopard for a few more weeks, so do I have to stay with XCode 4.0.2 until I move to Lion?
Yes, it seems:
This is a pre-release version of Xcode 4.1 for both Mac and iOS development. This release requires Mac OS X Lion and includes iOS SDK 4.2. Continue to use Xcode 3.2.5 on a Snow Leopard partition if you plan to submit Mac or iOS apps to the App Store
UPDATE: This question was asked in Jul 2011. As of now, Nov 2011, Xcode 4.2 is already released and made available on the Mac App store.
Yes. Xcode 4.1 Requires Lion. This has been well documented, but 4.2 is apparently available for Snow Leopard as well.
Xcode 4.2 is available for both Lion and Snow Leopard, if you download it from the iOS Developer Centre while logged in as a registered developer. The Mac App Store will only upgrade you to Xcode 4.2 if you upgrade to Lion, but you can manually install it on Snow Leopard by manually downloading it as described here, and it runs just fine on Snow Leopard.
Yes moving to Lion is a requirement. 4.1 brings all of the Lion features into the Xcode environment. I would say if you aren't ready to move to Lion yet, then just stick with 4.0.2 unless you need to develop for Lion.
UPDATE: As of Xcode 4.2, it is now for both Lion and Snow Leopard.
来源：https://stackoverflow.com/questions/6838774/is-moving-to-lion-a-prerequisite-for-moving-to-xcode-4-1-from-the-app-store